1. Field of the Invention
The present invention relates to a high voltage power supply apparatus used in an image forming apparatus and the image forming apparatus provided with the high voltage power supply apparatus, and more particularly, to a high voltage power supply apparatus used in an image forming apparatus using an electrophotographic system, and an image forming apparatus provided with the high voltage power supply apparatus and using the electrophotographic system.
2. Related Background Art
As an example of a high voltage power supply apparatus in a conventional image forming apparatus using the electrophotographic system, a configuration of the image forming apparatus, a configuration of the high voltage power supply apparatus used in the image forming apparatus and a circuit configuration of a developing high voltage power supply will be explained in order.
FIG. 7 shows a conventional type of an image forming apparatus, and a configuration of a color image forming apparatus of a four drum system.
A recording medium 110 fed by a pickup roller 111 is first detected at its tip position by a registration sensor 112, and then conveyed by conveying roller pairs 113, 114 and a conveying belt 105. Scanner units 100a to 100d successively irradiate a laser beam on photosensitive drums 101a to 101d in accordance with the detection timing of the registration sensor 112. At this time, photosensitive drums 101a to 101d, which are charged by electrostatic charge rollers 104a to 104d, have an electrostatic latent image formed thereon by the laser beam irradiation, and further have a toner image formed thereon by developing devices 102a to 102d and developing sleeves 103a to 103d. Then, the toner image is transferred to the recording medium 110 conveyed on the conveying belt 105 by transfer rollers 106a to 106d. Then, the recording medium 110 is conveyed to a fixing device 107, and outputted after the image is fixed. Here, the English character “a” of each of the reference characters denotes a configuration and unit for a color of cyan, “b” denotes a configuration and unit for a color of magenta, “c” denotes a configuration and unit for a color of yellow, and “d” denotes a configuration and unit for a color of black.
Next, a configuration of a high voltage power supply apparatus in the image forming apparatus shown in FIG. 7 is explained using FIG. 8.
The image forming apparatus comprises four kinds of high voltage power supply apparatuses including electrostatic charge bias high voltage power supply apparatuses 30a to 30d, which generate an electrostatic charge bias voltage, developing bias high voltage power supply apparatuses 31a to 31d, which generate a developing bias voltage, transfer bias high voltage power supply apparatuses 32a to 32d, which generate a transfer bias voltage, and transfer reverse bias high voltage power supply apparatuses 33a to 33d, which generate a transfer reverse bias voltage. The electrostatic charge bias high voltage power supply apparatuses 30a to 30d form a background potential on the surface of the photosensitive drums 101a to 101d, by applying the electrostatic charge bias voltage to the electrostatic charge rollers 104a to 104d, thereby making the surface of the photosensitive drums set to a state that an electrostatic latent image can be formed by irradiation of a laser beam. The developing bias high voltage power supply apparatus 31a to 31d reciprocate the toner between the developing sleeves 103a to 103d and the photosensitive drums 101a to 101d which are separated from the developing sleeves 103a to 103d, respectively, by applying an AC voltage to the developing sleeves 103a to 103d, to thereby make a toner image formed on the electrostatic latent image. The transfer bias high voltage power supply apparatus 32a to 32d transfer the toner image to the recording medium 110, by applying the transfer bias voltage to the transfer rollers 106a to 106d. Further, the transfer reverse bias high voltage power supply apparatuses 33a to 33d return a waste toner on the conveying belt 105 to the photosensitive drums 101a to 101d, by applying the transfer reverse bias voltage to the transfer rollers 106a to 106d at the time of the cleaning operation of the conveying belt 105. Here, the waste toner returned to the photosensitive drums 101a to 101d is scraped off by cleaning blades 115a to 115d, so as to be stored in the waste toner containers 116a to 116d. 
Next, an example of a circuit configuration of the developing bias high voltage power supply apparatus 31a in the high voltage power supply apparatus shown in FIG. 8 is explained using FIG. 9.
As disclosed in Japanese Patent Application Laid-Open No. H10-28328, in a DC high voltage power supply 10, an AC voltage generated by a DC driving circuit 12 is stepped up by a transformer 13 to a voltage having an amplitude of several tens of times as large as that of the AC voltage, and then smoothed by a rectifying circuit 14, as a result of which, a DC voltage is outputted between outputs 20, 21. A detection circuit 15 detects the voltage between the outputs 20, 21. A DC control circuit 16 performs control so as to make the voltage between the outputs 20, 21 become a predetermined value determined by a DC control signal 28, on the basis of the detection result of the detection circuit 15. On the other hand, in an AC high voltage power supply 11, an AC pulse signal 29 is amplified by an AC driving circuit 17 and then stepped up by a transformer 18 to a voltage having an amplitude of several tens of times as large as that of the amplified AC pulse signal 29, as a result of which, an AC voltage is outputted between outputs 22, 23. Here, the outputs 20, 21 and the outputs 22, 23 are connected in series, so that a voltage consisting of the output voltage of the DC high voltage power supply 10 superposed on the output voltage of the AC high voltage power supply 11 is outputted to an output end 25.
Further, the adjustment of printing density is performed by changing the output voltage of the DC high voltage power supply 10 by means of the DC control signal 28. At this time, the amplitude of the output voltage of the AC high voltage power supply 11 is kept constant.
However, the above-described conventional circuit configuration has problems that the output voltage of the AC high voltage power supply 11 easily fluctuates during load fluctuation, because the output voltage is not detected and controlled, and because the so-called open control is used, and that it is difficult to achieve the high output voltage precision at the output end because the output voltage precision at the output end is determined by a combination of the precision of the DC high voltage power supply and the precision of the AC high voltage power supply.